Sphingomyelins contain phosphocholine
or phosphoethanolamine as their polar head group and
are therefore classified along with glycerophospholipids
as phospholipids. Indeed, sphingomyelins
resemble phosphatidylcholines in their general properties
and three-dimensional structure, and in having
no net charge on their head groups .
Sphingomyelins are present in the plasma membranes
of animal cells and are especially prominent in myelin, a
membranous sheath that surrounds and insulates the axons
of some neurons—thus the name “sphingomyelins.[1]

Discovered in the 1880s, sphingomyelin was first isolated by Johann L.W. Thudicum.[2] The structure of sphingomyelin was first reported in 1927 as N-acyl-sphingosine-1-phosphorylcholine.[2] Sphingomyelin content in mammals ranges from 2 to 15% in most tissues, with higher concentrations found in nerve tissues, red blood cells, and the ocular lenses. Sphingomyelin has significant structural and functional roles in the cell. It is a plasma membrane component and participates in many signaling pathways. The metabolism of sphingomyelin creates many products that play significant roles in the cell.[2]

Physical Characteristics

Composition

Sphingomyelin consists of a phosphocholine head group, a sphingosine, and a fatty acid. It is one of the few membrane phospholipids not synthesized from glycerol. The sphingosine and fatty acid can collectively be categorized as a ceramide. This composition allows sphingomyelin to play significant roles in signaling pathways: the degradation and synthesis of sphingomyelin produce important second messengers for signal transduction.

Sphingomyelin obtained from natural sources, such as eggs or bovine brain, contains fatty acids of various chain length. Sphingomyelin with set chain length, such as palmitoylsphingomyelin with a saturated 16 acyl chain, is available commercially.[3]

Properties

Sphingomyelin have a nearly cylindrical shape, however many have a significant chain mismatch (the lengths of the two hydrophobic chains are significantly different).[4] The hydrophobic chains of sphingomyelin tend to be much more saturated than other phospholipids. The main transition phase temperature of sphingomyelins is also higher compared to the phase transition temperature of similar phospholipids, near 37 C. This can introduce lateral heterogeneity in the membrane, generating domains in the membrane bilayer.[4]

Sphingomyelin undergoes significant interactions with cholesterol. Cholesterol has the ability to eliminate the liquid to solid phase transition in phospholipids. Due to sphingomyelin transition temperature being within physiological temperature ranges, cholesterol can play a significant role in the phase of sphingomyelin. Sphingomyelin are also more prone to intermolecular hydrogen bonding than other phospholipids.[5]

Location

Sphingomyelin is synthesized at the endoplasmic reticulum (ER), where it can be found in low amounts, and at the trans Golgi. It is enriched at the plasma membrane with a greater concentration on the outer than the inner leaflet.[6] The Golgi complex represents an intermediate between the ER and plasma membrane, with slightly higher concentrations towards the trans side.[7]

Metabolism

Synthesis

The synthesis of sphingomyelin involves the enzymatic transfer of a phosphocholine from phosphatidylcholine to a ceramide. The first committed step of sphingomyelin synthesis involves the condensation of L-serine and palmitoyl-CoA. This reaction is catalyzed by serine palmitoyltransferase. The product of this reaction is reduced, yielding dihydrosphingosine. The dihydrosphingosine undergoes N-acylation followed by desaturation to yield a ceramide. Each one of these reactions occurs at the cytosolic surface of the endoplasmic reticulum. The ceramide is transported to the Golgi apparatus where it can be converted to sphingomyelin. Sphingomyelin synthase is responsible for the production of sphingomyelin from ceramide. Diacylglycerol is produced as a byproduct when the phosphocholine is transferred.[8]

Degradation

Sphingomyelin breakdown is responsible for initiating many universal signaling pathways. It is hydrolyzed by sphingomyelinases (sphingomyelin specific type-C phosopholipases).[6] The phosphocholine head group is released into the aqueous environment while the ceramide diffuses through the membrane.

Function

Membranes

The membranous myelin sheath that surrounds and electrically insulates many nerve cell axons is particularly rich in sphingomyelin, suggesting its role as an insulator of nerve fibers.[1] The plasma membrane of other cells is also abundant in sphingomyelin, though it is largely to be found in the exoplasmic leaflet of the cell membrane. There is, however, some evidence that there may also be a sphingomyelin pool in the inner leaflet of the membrane.[9][10] Moreover, neutral sphingomyelinase-2 – an enzyme that breaks down sphingomyelin into ceramide - has been found to localise exclusively to the inner leaflet, further suggesting that there may be sphingomyelin present there.[11]

Signal Transduction

The function of sphingomyelin remained unclear until it was found to have a role in signal transduction.[12] It has been discovered that sphingomyelin plays a significant role in cell signaling pathways. The synthesis of sphingomyelin at the plasma membrane by sphingomyelin synthase 2 produces diacylglycerol, which is a lipid-soluble second messenger that can pass along a signal cascade. In addition, the degradation of sphingomyelin can produce ceramide which is involved in the apoptotic signalling pathway.

Apoptosis

Sphingomyelin has been found to have a role in cell apoptosis by hydrolyzing into ceramide. Studies in the late 1990s had found that ceramide was produced in a variety of conditions leading to apoptosis.[13] It was then hypothesized that sphingomyelin hydrolysis and ceramide signaling were essential in the decision of whether a cell dies. In the early 2000s new studies emerged that defined a new role for sphingomyelin hydrolysis in apoptosis, determining not only when a cell dies but how.[13] After more experimentation it has been shown that if sphingomyelin hydrolysis happens at a sufficiently early point in the pathway the production of ceramide may influence either the rate and form of cell death or work to release blocks on downstream events.[13]

Lipid Rafts

Sphingomyelin, as well as other sphingolipids, are associated with lipid microdomains in the plasma membrane known as lipid rafts. Lipid rafts are characterized by the lipid molecules being in the lipid ordered phase, offering more structure and rigidity compared to the rest of the plasma membrane. In the rafts, the acyl chains have low chain motion but the molecules have high lateral mobility. This order is in part due to the higher transition temperature of sphingolipids as well as the interactions of these lipids with cholesterol. Cholesterol is a relatively small, nonpolar molecule that can fill the space between the sphingolipids that is a result of the large acyl chains. Lipid rafts are thought to be involved in many cell processes, such as membrane sorting and trafficking, signal transduction, and cell polarization.[14] Due to the specific types of lipids in these microdomains, lipid rafts can accumulate certain types of proteins associated with them, thereby increasing the special functions they possess. Lipid rafts have been speculated to be involved in the cascade of cell apoptosis.[15]

Abnormalities and Associated diseases

Sphingomyelin can accumulate in a rare hereditary disease called Niemann-Pick Disease, types A and B. It is a genetically-inherited disease caused by a deficiency in the enzymesphingomyelinase, which causes the accumulation of sphingomyelin in spleen, liver, lungs, bone marrow, and brain, causing irreversible neurological damage. Of the two types involving sphingomyelinase, type A occurs in infants. It is characterized by jaundice, an enlarged liver, and profound brain damage. Children with this type rarely live beyond 18 months. Type B involves an enlarged liver and spleen, which usually occurs in the pre-teen years. The brain is not affected. Most patients present with <1% normal levels of the enzyme in comparison to normal levels.

As a result of the autoimmune disease multiple sclerosis (MS), the myelin sheath of neuronal cells in the brain and spinal cord is degraded, resulting in loss of signal transduction capability. MS patients exhibit upregulation of certain cytokines in the cerebrospinal fluid, particularly tumor necrosis factor alpha. This activates sphingomyelinase, an enzyme that catalyzes the hydrolysis of sphingomyelin to ceramide; sphingomyelinase activity has been observed in conjunction with cellular apoptosis.[16]